This invention generally relates to the manufacture of hydrogen sensors and, more particularly, to the use of thick film printing processes to print palladium paste and other conductive and protective layers onto a substrate to manufacture resistance-type hydrogen sensors. This invention also includes the product of the manufacturing process.
Various types of electronic assemblies, manufacturing processes, machines and other devices utilize, produce, or are harmed by the presence of hydrogen gas. Therefore, a number of sensing devices have been developed to detect and measure the presence of hydrogen. One desirable characteristic of such sensing devices is to detect hydrogen at low concentrations, around a 1% to about 3% hydrogen in the atmosphere. Sensing at low concentrations is desirable first to minimize the harmful effects of hydrogen (i.e., metal embrittlement, etc.) and second, to provide better anticipation to avoid explosive mixtures (hydrogen reaches an explosive concentration at about 4%).
A variety of techniques to sense the presence of hydrogen have been developed over the years including various methods discussed in U.S. Pat. No. 5,451,920 (referred to herein as the "'920 patent") which is incorporated herein by reference. In particular, thin and thick film hydrogen sensors have been developed which utilize palladium (Pd) metal films to form a resistance network. Palladium metal changes in its electrical resistance characteristics as it is exposed to hydrogen. Thus, changes in the electrical characteristics of such a network can be utilized to sense the presence of hydrogen.
Typically, such sensing networks are set up to form a Wheatstone Bridge or the like wherein one or two of the four legs of the bridge are formed from palladium which is exposed to the gas to be sensed and two legs of the bridge are palladium which has been encapsulated so as to protect those legs from exposure to such gas. Additional elements are added to the bridge to form balancing resistors so that the bridge preferably may be balanced in all four quadrants when no hydrogen is being detected.
Existing hydrogen sensors made by thick film processes such as are discussed in the '920 patent have several limitations. Perhaps the most crucial is that these sensors have a tendency to be inaccurate and not reliably sense hydrogen at concentrations below a 4% concentration level. Moreover, the sensors have a tendency to drift or degrade with time. Some of these characteristics appear related to problems experienced in the process of applying a passivation layer, i.e., the process of encapsulating portions of the palladium to prevent hydrogen from reaching portions of the network. Another notable limitation of processes such as described in the '920 patent is that such processes historically produce relatively low yields of useable sensors and the process repeatability is lacking, due apparently to problems in reliably achieving adherence of various portions of the thick film network to the substrate involved.